The present invention relates to multiple-access communication systems, and more particularly to a multiple-access communication system that utilizes a direct-sequence code-division multiple-access (DS-CDMA) approach, thereby allowing a multiplicity of separate transmitters to efficiently access a stationary base-station receiver.
Multiple-access communication systems are typically designed for use with a relatively large number of separate transmitters (e.g., portable or mobile transmitters) that interface with at least one stationary receiver at one or more designated receiving locations. Such multi-access systems are commonly used with digital cellular telephones, personal communication services, wireless local area networks (LAN's), and the like. Because the receiver(s) of such systems must allow access to a large number of users, each having a transmitter, some means must be used to efficiently interconnect the multiple transmitters to the receiver(s), i.e., to efficiently utilize the available channel spectrum. Common techniques used to allow such multiple access include frequency division multiple access (FDMA), time division multiple access (TDMA), and code-division multiple access (CDMA). The present invention provides multiple access through a CDMA-based communication system.
A simplified model of a CDMA system is as follows: a common carrier frequency, modulated with data having a known bit time, is transmitted to a common receiver from each of several transmitters. All of the transmitters share the same carrier frequency. Thus, all of the transmitters may be tuned to the same RF frequency. Each transmitter has its own low bandwidth information bearing signal having a prescribed data rate or bit rate. This low bandwidth information signal is multiplied by a unique high bandwidth signature waveform, which makes it possible for the receiver to distinguish the desired signal from the other signals transmitted from the other transmitters. For purposes of the present application, it is assumed that the signature waveform consists of a sum of time-offset copies of a waveform called the "chip waveform." The signature waveform may be visualized as the result of convolving the chip waveform with a train of impulses, each of unit area and of positive or negative polarity. The sequence of polarities included within the train of impulses is known in the art as a "spreading sequence" (or "spreading code"). The spreading sequence is unique to each transmitter, but the chip waveform is the same. If, as is common in the art, the spreading sequences appear statistically like random binary sequences, then the power spectrum of the resulting signature sequence is substantially the same as that of the original chip waveform. Thus, each transmitter sends a waveform of similar power spectrum over the channel, and a receiver which has knowledge of a spreading sequence used by a transmitter can distinguish the signal sent by that transmitter on that basis. The unit of time between impulses in the impulse train described above is known as a "chip time", T.sub.c and the reciprocal of this time is known as the "chip rate". The present invention is not limited to the above model of DS-CDMA, but the description is simplified by such a model.
A common problem facing all multiple-access communication systems is accurately detecting the transmitted signal at the receiver after the signal has passed through a noisy channel, i.e., after the transmitted signal has been corrupted with noise. Such noise may include signals from other transmitters, thermal noise, or noise from other sources. A measure of the ability to accurately detect a signal in a noisy environment is the signal-to-noise ratio (SNR), defined as the ratio of the power of the desired signal divided by the power of all other undesired signals, measured at the final signal which is used to make a decision about the information bearing signal. A high SNR indicates that the integrity of the signal, when received at the receiver, has been more or less preserved, thereby enabling the individual bits of the signal to be detected above the noise with a low probability of error. It is thus a common objective of any communication system, including multiple-access communication systems, to achieve a high SNR, despite the noisy channels and mediums through which the transmitted signal may traverse as it propagates from the transmitter to the receiver.
In a DS-CDMA system, the transmitted signal is not only subject to additive white Gaussian noise (AWGN), a common form of noise in most communication channels, but is also subject to "multiple-access noise", i.e., noise resulting from the presence of other users who are transmitting at the prescribed carrier frequency and bit rates, but with different signature waveforms. To minimize the effect of the AWGN, it is known in the art to implement the receiver of a DS-CDMA system as a filter matched to the signature waveform of the user (transmitter) of interest. Unfortunately, such matched filter, while optimum for minimizing probability of bit error in AWGN, performs poorly when significant multiple-access noise is present. Thus, what is needed is a type of filter for use within a DS-CDMA receiver that performs acceptably in the presence of significant multiple-access interference.
An optimum multi-user receiver that minimizes multi-user noise is known. However, such optimum multi-user receiver is extremely complex. Numerous sub-optimal simplifications of such optimal structure have been proposed, however, such "simplifications" still require locking and despreading of some or all of the interfering signals, and hence also represent substantially complex circuitry. Thus, the matched filter receiver, despite its limitations, represents the most common method in practice.
Another form of channel distortion frequently encountered with multiple-access communication systems are signal reflections, or having the same signal traverse multiple paths as it arrives at the receiver. Channel distortion caused by multiple paths is commonly dealt with in the prior art by utilizing a RAKE receiver. The term "RAKE" receiver has been coined in the art, to describe the finite length impulse train which results from such receiver when an impulse is applied to the input. The impulse response, when graphed, looks like the teeth of a garden rake.) A RAKE receiver includes a delay line, or a series of delay elements, with signal taps being provided after each delay. The tapped signals are then appropriately combined with other signals, through feedback and feedforward connections, in order to minimize the effects of channel distortion.
The main source of noise in a typical CDMA system is multiple-access noise, which has a power spectrum similar to that of the signal of interest. Thus, a RAKE receiver must have signal taps spaced such that this power spectrum is flat (uncorrelated) when aliased at the tap rate. Such a limitation may significantly encumber the operation of the RAKE receiver, especially when there is significant excess bandwidth (i.e., a significantly non-flat transmitted power spectrum). This is especially likely to be the case for high bandwidth CDMA systems, owing to the difficulty of generating a flat-spectrum chip waveform at very high rates. What is needed, therefore, is an improved receiver that performs the function of the RAKE receiver, thereby compensating for channel distortion, but which does so without the tap spacing limitations of the prior an RAKE receivers.